This invention relates to a thermoforming apparatus and a method of controlling the motion of a mold-carrying platen by means of a cam disc. The thermoforming apparatus has a mechanism for receiving a reel of a thermoplastic sheet, a cyclically operating sheet feeding mechanism, a heating arrangement for heating portions of the thermoplastic sheet and a mold station in which the heated sheet portions are sequentially deformed by means of a differential pressure. The mold station includes a cam disc as well as an associated drive to effect the reciprocating motion of the mold-carrying platen. The mold station may be so designed that it includes stamping tools so as to simultaneously serve as a stamping station. It is, however, feasible to arrange a separate stamping station downstream of the thermoforming apparatus for removing the shaped article from the continuous thermoplastic sheet.
The cam disc may drive the mold platen directly from below or it may be connected with the platen by means of a linkage drive which then imparts the reciprocating motion to the platen. This latter alternative is disclosed, for example, in U.S. Pat. No. 3,816,052.
The cam disc is usually driven by means of a variable speed drive to set the rpm of the cam disc and thus predetermine the cycle of the thermoforming apparatus and to adapt it to the mold in a stepless manner.
Prior to discussing the disadvantages of the above-outlined conventional operational method, first the steps within one operational cycle will be set forth in more detail.
Subsequent to introducing a heated sheet portion into the mold station, the platen, together with the lower mold half, is raised and the sheet is tensioned between the two mold halves. The article to be manufactured is formed by introducing into the mold pressurized air which causes the thermoplastic sheet material to conform to the mold wall. This condition should be maintained for a certain period of time to ensure that the deformed sheet can cool in the deformed position. Thereafter, the platen is moved downwardly. This motion may be preceded by a short upward movement, during which the article is stamped out of the sheet. In the open position of the mold, the next sheet portion is fed into the mold station. Thereafter, the next operational cycle begins.
During one cycle of operation, the following significant consecutive time periods occur:
(a) duration of the lifting stroke of the platen;
(b) dwelling period in the upper position of the platen for deformation and cooling;
(c) duration of the return stroke of the platen; and
(d) dwelling period in the lower position of the platen during the advance of the thermoplastic sheet.
The dwelling period identified under (b) is in a large measure dependent on the type of the thermoplastic sheet, the sheet thickness, the mold temperature and the shape of the mold. The periods identified under (a) and (d) are essentially independent of the properties of the article, that is, for a particular thermoforming apparatus minimum periods can be assigned. They depend essentially on the established output, the component masses to be accelerated and braked, the platen stroke and the sheet feed. The period identified under (c) for lowering the platen may be dependent on the article, at least at the beginning of the downward motion, since in case of certain shapes (vertical walls, thin walls or undercut portions) it is not feasible to move the mold downwardly at an arbitrarily high speed because the article may be damaged. At the end of the downward motion, the downwardly directed velocity is again dependent from the masses which have to be braked.
In case of articles which require a long dwelling period (b) and/or a slow return stroke (c) of the platen, it has been necessary to set the rpm of the cam disc drive at a low value. This necessarily has resulted in a small cycle number, because the periods (a) and (d) were necessarily also lengthened, although such a lengthening of these two last-named periods is not needed either from the point of view of the apparatus or the product manufactured.
In order to eliminate the above-discussed disadvantage, it has been heretofore known only to drive the cam disc by means of a brake-and-clutch combination and to stop the cam disc in the upper position of the platen for a short period. In this manner, to be sure, an extension of the dwelling period could be achieved. This method, however, is not adapted to also reduce the speed of the return motion of the platen so that, at least in case of products that require a slow deformation, a low rpm has to be set for the cam disc. Further, the braking of the cam disc and its restarting to a cycle of approximately 30/min or above has caused a significant wear of the brake and clutch. This has required the replacement of these components a relatively short service life. All this has resulted in long maintenance periods and high costs.